Chapter 9 – Inheritance Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved.

Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved. Inheritance Hierarchies

Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved. Inheritance is a mechanism for extending existing classes by adding instance variables and methods: class SavingsAccount extends BankAccount { added instance variables new methods } A subclass inherits the methods of its superclass : SavingsAccount collegeFund = new SavingsAccount(10); // Savings account with 10% interest collegeFund.deposit(500); // OK to use BankAccount method with SavingsAccount object Inheritance Hierarchies

Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved. In subclass, specify added instance variables, added methods, and changed or overridden methods: public class SavingsAccount extends BankAccount { private double interestRate; public SavingsAccount(double rate) { Constructor implementation } public void addInterest() { Method implementation } Inheritance Hierarchies

Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved. Instance variables declared in the superclass are present in subclass objects SavingsAccount object inherits the balance instance variable from BankAccount, and gains one additional instance variable, interestRate : Inheritance Hierarchies

Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved. Implement the new addInterest method: public class SavingsAccount extends BankAccount { private double interestRate; public SavingsAccount(double rate) { interestRate = rate; } public void addInterest() { double interest = getBalance() * interestRate / 100; deposit(interest); } Inheritance Hierarchies

Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved. A subclass has no access to private instance variables of its superclass Encapsulation: addInterest calls getBalance rather than updating the balance variable of the superclass (variable is private ) Note that addInterest calls getBalance without specifying an implicit parameter (the calls apply to the same object) Inheriting from a class differs from implementing an interface: the subclass inherits behavior from the superclass Inheritance Hierarchies

Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved. 1 /** 2 An account that earns interest at a fixed rate. 3 */ 4 public class SavingsAccount extends BankAccount 5 { 6 private double interestRate; 7 8 /** 9 Constructs a bank account with a given interest rate. rate the interest rate 11 */ 12 public SavingsAccount(double rate) 13 { 14 interestRate = rate; 15 } 16 ch09/accounts/SavingsAccount.java Continued

Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved. 17 /** 18 Adds the earned interest to the account balance. 19 */ 20 public void addInterest() 21 { 22 double interest = getBalance() * interestRate / 100; 23 deposit(interest); 24 } 25 } ch09/accounts/SavingsAccount.java (cont.)

Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved. Syntax 9.1 Inheritance

Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved. Which instance variables does an object of class SavingsAccount have? Answer: Two instance variables: balance and interestRate. Self Check 9.3

Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved. Name four methods that you can apply to SavingsAccount objects. Answer: deposit, withdraw, getBalance, and addInterest. Self Check 9.4

Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved. Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved. If the class Manager extends the class Employee, which class is the superclass and which is the subclass? Answer: Manager is the subclass; Employee is the superclass. Self Check 9.5

Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved. A subclass has no access to the private instance variables of the superclass: public class SavingsAccount extends BankAccount { public void addInterest() { double interest = getBalance() * interestRate / 100; balance = balance + interest; // Error }... } Common Error: Shadowing Instance Variables

Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved. Beginner’s error: “solve” this problem by adding another instance variable with same name: public class SavingsAccount extends BankAccount { private double balance; // Don’t public void addInterest() { double interest = getBalance() * interestRate / 100; balance = balance + interest; // Compiles but doesn’t // update the correct balance }... } Common Error: Shadowing Instance Variables

Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved. Now the addInterest method compiles, but it doesn’t update the correct balance! Common Error: Shadowing Instance Variables

Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved. A subclass method overrides a superclass method if it has the same name and parameter types as a superclass method When such a method is applied to a subclass object, the overriding method is executed Overriding Methods

Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved. Example: deposit and withdraw methods of the CheckingAccount class override the deposit and withdraw methods of the BankAccount class to handle transaction fees: public class BankAccount {... public void deposit(double amount) {... } public void withdraw(double amount) {... } public double getBalance() {... } } public class CheckingAccount extends BankAccount {... public void deposit(double amount) {... } public void withdraw(double amount) {... } public void deductFees() {... } } Overriding Methods

Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved. Use the super reserved word to call a method of the superclass: public class CheckingAccount extends BankAccount { public void deposit(double amount) { transactionCount++; // Now add amount to balance super.deposit } Overriding Methods

Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved. Remaining methods of CheckingAccount also invoke a superclass method: public class CheckingAccount extends BankAccount { private static final int FREE_TRANSACTIONS = 3; private static final double TRANSACTION_FEE = 2.0; private int transactionCount;... public void withdraw(double amount { transactionCount++; // Now subtract amount from balance super.withdraw(amount); } Overriding Methods Continued

Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved. public void deductFees() { if (transactionCount > FREE_TRANSACTIONS) { double fees = TRANSACTION_FEE * (transactionCount - FREE_TRANSACTIONS); super.withdraw(fees); } transactionCount = 0; }... } Overriding Methods (cont.)

Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved. Syntax 9.2 Calling a Superclass Method